It has long been known that exhaust gas catalyzers convert toxic gases at a rate which decreases with increasing deterioration of the catalyzer. It has been shown that the signals of lambda probes mounted forward and rearward of the catalyzer, respectively, are suitable to determine the converting capacity of the catalyzer. In U.S. Pat. No. 3,962,866, the difference of the two above-mentioned probes is formed. When the difference drops below a threshold value, a warning signal is emitted which indicates that the catalyzer should be exchanged. In U.S. Pat. No. 4,622,809, a corresponding warning signal is provided when the amplitude of the signal of the probe rearward of the catalyzer exceeds a threshold value or the mean value leaves a pregiven range even though an effort is made to control to a minimum amplitude and to maintain the range. For this purpose, control parameters are changed in dependence upon the signal from the probe rearward of the catalyzer.
It is apparent that the signals of the probe rearward of the catalyzer are not only dependent upon the state of deterioration of the catalyzer but are also dependent upon the amplitude and upon the time-dependent trace of the signal of the probe forward of the catalyzer, that is, from the oxygen quantity which flows into the catalyzer or the oxygen quantity taken from the catalyzer. For precise data of the conversion capacity of a catalyzer, it is therefore necessary to optimize the conditions for the probe signals at which the catalyzer is judged to be no longer useable.
For optimization problems, it is well known that it is helpful if the system to be optimized can be simulated. In this way, many different time sequences can be investigated using computer techniques without complex experiments being necessary.